The invention concerns a method for descaling a metal strip, especially a hot-rolled strip of normal steel or a hot-rolled or cold-rolled strip of austenitic or ferritic stainless steel, in which the metal strip is guided in a direction of conveyance through at least one plasma descaling unit, in which it is subjected to a plasma descaling. The invention also concerns a device for descaling a metal strip.
Steel strip must have a scale-free surface before it can be further processed, e.g., by cold rolling, by the application of a metallic coating, or by direct working into a finished product. Therefore, the scale that forms, for example, during hot rolling and the subsequent cooling phase must be completely removed. In previously known methods, this is accomplished by a pickling process, in which, depending on the grade of steel, the scale, which consists of various iron oxides (FeO, Fe3O4, Fe2O3) or, in the case of stainless steels, of chromium-rich iron oxides, is dissolved by means of various acids (e.g., hydrochloric acid, sulfuric acid, nitric acid, or mixed acid) at elevated temperatures by chemical reaction with the acid. Before the pickling operation, an additional mechanical treatment by stretcher-and-roller leveling is necessary in the case of normal steel to break up the scale to allow faster penetration of the acid into the layer of scale. In the case of stainless, austenitic, and ferritic steels, which are much more difficult to pickle, an annealing operation and a preliminary mechanical scaling operation must be performed on the strip before the pickling process is carried out in order to produce a strip surface that can be pickled as well as possible. After the pickling operation, to prevent oxidation, the steel strip must be rinsed, dried, and, depending on requirements, oiled. The pickling of steel strip is carried out in continuous lines, whose process section can be very long, depending on the strip speed. Therefore, installations of this type require very large investments. In addition, the pickling process uses a tremendous amount of power and entails great expense for the elimination of wastewater and the regeneration of the hydrochloric acid, which is the type of acid usually used for normal steel.
Due to these disadvantages, the prior art also includes various approaches for accomplishing the descaling of metal strands without the use of acids. Previous developments along these lines are generally based on mechanical removal of the scale (e.g., the Ishiclean method, the APO method). However, with respect to their economy and the quality of the descaled surface, methods of these types are not suitable for the industrial descaling of wide steel strip. Therefore, acids continue to be used for descaling this type of strip.
Consequently, so far it has been necessary to accept the disadvantages with respect to economy and environmental pollution.
Recent approaches to the descaling of metal strands have been based on plasma technology. Methods and devices of the aforementioned type for descaling metal strands with different geometries, for example, metal strip or metal wire, are already well known in various forms in the prior art. Reference is made, for example, to WO 2004/044257 A1, WO 2000/056949 A1 and RU 2 145 912 C1. In the plasma descaling technology disclosed in the cited documents, the material to be descaled runs between special electrodes located in a vacuum chamber. The descaling is effected by the plasma produced between the steel strip and the electrodes, and the result is a bare metallic surface with no residue. Plasma technology thus represents an economical, qualitatively satisfactory and environmentally friendly possibility for descaling and cleaning steel surfaces. It can be used for normal steel as well as for stainless, austenitic, and ferritic steels. No special pretreatment is necessary.
In plasma descaling, the strip thus runs through a vacuum chamber between electrodes arranged above and below the strip. The plasma is located between the electrodes and the surface of the strip on both sides of the strip. The action of the plasma on the scale results in the removal of the oxides on the surface of the strip, and this is associated with an increase in the temperature of the strip, which can be a serious disadvantage. The temperature increase can result in the formation of an oxide film on the surface of the strip when the descaled strip emerges from the vacuum and enters the air. An oxide film is unacceptable for further processing steps, such as cold rolling or the direct working of hot strip.
Various proposals have been made to improve this situation by cooling the metal strip following the plasma descaling. Methods of this type are disclosed, for example, in JP 07132316 A, JP 06279842 A, JP 06248355 A, JP 0312034, JP 2001140051 A, and JP 05105941 A. However, the concepts disclosed in this literature are aimed at cooling measures that are associated with considerable disadvantages in some cases or are relatively inefficient. For example, a cooling medium is sprayed, which makes it necessary to carry out a subsequent drying of the metal strip. If the metal strip is treated with a cooling gas, the cooling rate is very low, and, in addition, a solution of this type is not possible in a vacuum. The other proposed solutions offer almost no possibility of realizing a specific temperature program for the metal strip.
For most applications, controlled cooling of the metal strip during or after the descaling is necessary before the strip comes into contact with air. Systematic cooling of this type is not possible with the prior-art solutions.